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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

SOAPS

461

in

acid and absorb the carbon dioxide liberated in caustic potash solution
the ordinary manner.1

7.  Free Fatty Acids.-These are determined only when the soap does
not exhibit an alkaline reaction.

20 grams of the soap are dissolved in neutral 60% alcohol and the
solution titrated with alcoholic decinormal caustic potash in presence of
phenolphthalein. The acidity is expressed as oleic acid: i c.c. N/io-KOH
= 0-0282 gram of oleic acid.

8.  Neutral Fat and  Unsaponifiable Substances.—From 6 to 8
grams of the total fat, extracted as in 3, are dissolved in about 50 c.c. of
96% alcohol and the liquid neutralised with seminormal alcoholic caustic
potash in presence of phenolphthalein (to a faint pink coloration); it is
then diluted with about 50 c.c. of water and extracted successively with
100 c.c., 50 c.c., and 50 c.c. of petroleum ether (b.pt. not above 65°).   The
united petroleum ether solutions are washed three times with three quantities
of 10-20 c.c. of 58% alcohol and then evaporated, the residue being dried
at 100-105" and weighed.

This gives the neutral fat plus any unsaponifiable substances contained
in the soap.

In presence of the latter (which may be detected by a separate test),
the weighed residue must be saponified with alcoholic potash, the solution
extracted with petroleum ether in the manner described above, and the
new extract, representing the unsaponifiable substances, weighed. The
neutral fat is then given by difference.

9.  Resin.—The presence of resin  (colophony) in a soap is readily
detected by the application of Morawski's reaction (see p. 390) to the fatty
acids obtained from the soap itself, provided the latter does not contain
wool fat, in which case the test is made on the fatty acids after elimination
of the tmsapcmifiable matter.

The quantitative determination of the resin is effected by TwitchelTs
method (we p. 390)

10.  Glycerine.—-This occurs in "cold" soaps and in soft or potash
soaps (3-5%), in which its presence is due to the method of preparation,
and also in transparent or glycerine soaps, to which it is purposely added.

For its determination, 20-25 grams of the soap are dissolved in hot
water, decomposed by means of dilute sulphuric acid and filtered to remove
the. fatty acids. The filtrate is neutralised, defecated swith lead acetate,
made up to a definite volume and filtered, the glycerine being estimated in
an aliquot part by the dichromate method (see Glycerine).

If the. soap contains ethereal oils, sugar, dextrin or other substance
oxidisablc by dichromate, this method is inapplicable. In such cases the
defecated aqueous liquid is evaporated with addition of lime and the glycerine
then extracted with alcohol and ether in the manner described for the
determination of glycerine in wine.

11.  Nature of the Constituent Fats.—The external characters oi
soaps give some indication of the nature of the fatty matters used in the
manufacture.    White and mottled soaps are mostly prepared from tallow

1 See last part of preceding foot-note.mately N/40 solution) and the titer,